The present invention is directed to processes for producing mechanical pulps, and more particularly to hydrogen peroxide bleaching of thermomechanical pulps and the resultant pulps made therefrom.
Mechanical pulping is a process of mechanically triturating wood into its fibers for the purpose of making pulp. Mechanical pulping is attractive as a method for pulping because it achieves high yields when compared to chemical pulping because lignin is not removed from mechanically pulped woods, meaning scarce resources are more efficiently utilized. Pulps made using any of the conventional mechanical pulping methods are mainly used for newsprint, and are unsuitable for higher quality or more durable paper and products. This is due, in part, to the fact that mechanical pulps are generally more difficult to bleach than chemical pulps.
There are many variants of mechanical pulping including stone grinding (SG), pressurized stone grinding (PSG), refiner mechanical pulping (RMP), thermomechanical pulping (TMP), and chemi-thermomechanical pulping (CTMP). The latter three can further be grouped generally under refiner pulping processes. In RMP, wood chips are ground between rotating metal disks. The process usually is carried out in two stages. The first stage is mainly used to separate the fibers, while the second stage is used to treat the fiber surface for improved fiber bonding of paper products. In RMP, the wood chips are refined at atmospheric pressure in both a first and a second stage refiner. The refiner process generates heat by the friction of the metal disks against the wood. The heat is liberated as amounts of steam which is often used to soften the incoming chips.
TMP differs from RMP in that the pulp is made in a pressurized refiner. In this process, two stages are normally used also. The first stage refiner operates at elevated temperature and pressure, and the second stage refiner is at ambient conditions. The first stage separates the fibers and the second stage then treats the fibers. Pulps made by TMP have high strength, which makes the TMP process the most favored mechanical pulping process. However, there is still room for improvements. The TMP process consumes high energy, and the pulp produced by the TMP process tends to be darker than most other pulps.
CTMP uses both chemical and thermal pretreatment for processing the wood chips into pulp. CTMP is a chemi-thermomechanical process that is similar to TMP, except that the chips are first pretreated with relatively small amounts of sodium hydroxide with hydrogen peroxide under elevated temperature and pressure prior to refining. The adjuvant chemicals make the separation of the cellulosic fibers much easier in the refiners.
The foregoing list is by no means exhaustive. There are innumerable combinations and variants of the pulping processes as exemplified in The Handbook of Pulping and Papermaking, 2d ed., by Christopher J. Biermann, which is herein incorporated by reference. Of the mechanical pulping processes, the one which is considered by many in the field to be the most favorable, taking into consideration market conditions and environmental regulations, is the TMP process. However, were it not for the fact that chemi-thermomechanical pulping processes produce effluents of high color, high COD and BOD, which may be difficult to treat, CTMP processes would have an advantage over TMP processes because the energy grinding requirements for CTMP are about half that of TMP.
Bleaching is a term applied to a semi-chemical or chemical step in a pulping process to increase the brightness of both chemical and mechanical pulps. In mechanical pulping, the increase in brightness is achieved by altering the chemical structure of the conjugated double bonds in lignin. The conjugated double-bonded species are called chromophores. xe2x80x9cBrighteningxe2x80x9d is the term often used when referring to bleaching of mechanical pulps to distinguish it from the bleaching process of chemical pulps, which differs by removing lignin entirely. As used hereinafter xe2x80x9cbleachingxe2x80x9d will be intended to cover the process of xe2x80x9cbrighteningxe2x80x9d as well. In mechanical pulps, brightening is often carried out in a single step in the pulping process. The bleaching process is conventionally carried out in a bleaching train in one or a plurality of vessels (bleach towers or stages) in a distinct section of the mill plant, as opposed to the pulping section of the mill. Brightening can be carried out using oxidative and/or reductive chemical agents including oxidating reagents, such as hydrogen peroxide and reducing agents, such as dithionite, or sodium hydrosulfate. Normally, hydrogen peroxide, an oxidizing agent, is used with sodium hydroxide. For a more thorough discussion of bleaching chemistry, reference is made to Pulp Bleachingxe2x80x94Principles and Practice, by J. Ross Anderson and B. Amini; Section V: Chapter 1: Peroxide Bleaching of (Chemi)mechanical Pulps, by J. R. Presley and R. T. Hill. Sodium hydroxide is a strong alkali and provides the requisite high pH necessary to produce the active perhydroxyl ion, HOOxe2x88x92, thought to produce the bleaching effect in pulps. The cost of sodium hydroxide has been increasing due to changes in availability and energy costs. Concern over the environment has also meant a decrease in the available sodium hydroxide supply. Therefore, different alkali sources and different methods have been tried to find suitable alternatives for bleaching liquors and bleaching processes with limited success.
Hence, there is a need to improve existing mechanical pulping processes to provide higher brightness pulps by processes having added advantages.
When alkali peroxide bleaching at high temperatures, better brightness is obtained when using an alkali buffer (such as soda ash or magnesium hydroxide), instead of sodium hydroxide. Buffering the system at lower pH (about 9 to about 10.5) prevents peroxide decomposition and alkali darkening, but still provides adequate alkali to produce effective bleaching. The buffer releases alkalinity as necessary and provides just enough alkalinity for a slow, even production of perhydroxyl ions. The present invention provides a supply of perhydroxyl ions as needed for bleaching and prolongs the effective bleaching time, making the peroxide more effective and giving higher brightness and higher yields by reducing the breakdown of the wood fibers, thus overcoming many of the aforementioned problems.
A method of making bleached mechanical pulps is disclosed for pulping mills having a refining system. A step according to the invention is to provide a cellulosic material, such as wood chips, having an initial brightness level. A second step in the method in accordance with the invention is to introduce the cellulosic material to a refining system for conversion into a pulp. A third step in the method in accordance with the invention is to provide a bleaching liquor to the refining system, wherein the liquor comprises an amount of hydrogen peroxide and an amount of alkali, wherein up to 100% of alkali is either magnesium hydroxide, soda ash or a combination thereof. Any additional balance required to arrive at a suitable amount of alkali is supplied by NaOH. A fourth step in the method in accordance with the invention is to hold the pulp with the bleaching liquor at an effective temperature and for an effective time sufficient to increase the brightness of the pulp from the initial brightness level to brightness level equal to or higher than what can be obtained when 100% of alkali is NaOH and the pulp and bleaching liquor are contacted under about the same time and temperature conditions. Pulps having a brightness of at least 35 ISO or in the range of about 55 to 69.5 ISO are attainable by the methods of the present invention.
One embodiment uses a temperature in the range of about 85xc2x0 to about 160xc2x0 C. for about 2 to about 180 minutes, as the conditions under which the pulp and bleaching liquor are held. Another alternate second suitable temperature range includes greater than 100xc2x0 C. to about 160xc2x0 C. Three other alternate suitable time ranges include the ranges of from about 10 minutes to less than 180 minutes, or greater than 60 minutes to less than 120 minutes, or greater than 2 minutes to less than 60 minutes and the combination of these three alternate time ranges with the temperature ranges. Furthermore, any time or temperature range within the aforementioned time and temperature ranges can also be used.
In another alternate embodiment, a step of increasing the pH of the pulp to the range of about 9 to about 10.5 is provided, in addition to the steps mentioned above.
In another alternate embodiment, a method of making bleached mechanical pulps is disclosed for pulping mills having a refining system. A step according to the invention is to provide a cellulosic material having an initial brightness level. A second step in the method in accordance with the invention is to introduce a cellulosic material to a refining system for conversion to a pulp. A third step in the method in accordance with the invention is to provide a bleaching liquor to the refining system, wherein the liquor comprises a first amount of hydrogen peroxide and alkali, wherein up to 100% of alkali is magnesium hydroxide, soda ash, or a combination thereof. A fourth step in the method in accordance with the invention is to hold the pulp and the bleaching liquor at a temperature in the range of about 85xc2x0 C. to about 160xc2x0 C. for a time of about 2 to about 180 minutes. A fifth step in the method in accordance with the invention is to increase the brightness of the pulp about equal to or less than a brightness level which can be obtained if the bleaching liquor comprises a second amount of hydrogen peroxide which is greater than the first amount, wherein 100% of alkali is sodium hydroxide and the pulp and bleaching liquor are held under about the same time and temperature conditions.
A method of brightening TMP pulps in accordance with the invention provides significant advantages. The residual peroxide level is increased, meaning more effective use of hydrogen peroxide. A decrease in the oxalate concentration is noticed, meaning less scaling of process equipment, thereby reducing premature equipment wear. An increase in pulp yields is also realized. Furthermore, COD and BOD levels of plant effluents are reduced, which contribute to lower pollution levels entering waste water facilities.